Ambient noise frequency responsive audible vehicle alarm

ABSTRACT

A vehicle alarm system which has an ambient noise sensor, an alarm signal generator, and a system responsive to the sensor for varying the output frequency of the generator so as to produce a warning signal that has a frequency other than the frequency of the ambient noise.

United States Patent Inventor Herbert A. Raschke [51] Int. Cl G08b 23/00Greenbrae, Calif. [50] Field of Search 340/384; Appl. No. 813,786 179/1.2; 325/(lnquired) Filed Jan. 27, 1969 Patented May 18, 1971 ReferencesClted Assignee E. D. Bullard Company UNITED STATES PATENTS a F 3,160,87712/1964 Monomakhoff 340/384(E) commuatwn-m'part of applicant)" 3,173,1363/1965 Atkinson 340/3 84(E) 571,779, Aug. 11, 1966, now abandoned.

Przmary Exammer-R1chard Murray Attorney Townsend and Townsend ABSTRACT:A vehicle alarm system which has an ambient RESPONSWE noise sensor, analarm signal generator, and a system respon- 6 Cl 4 D sive to the sensorfor varying the output frequency of the rawmg generator so as to producea warning signal that has a frequen- U.S. Cl 340/384E cy other than thefrequency of the ambient noise.

I /1 /\71 75F 'V: Mr 727 E T 28 VA RIA BLE 26 I SAWTOOTH FREQUENCY IGENERATOR AUDIO I OSCILLATOR 1 1 1 34 I ILI'L I I DIFFERENTIATOR I 32 VI4 22 I I BIS TABL E MULTIVIBRATOR I2 I I STORAGE I 38- L 4 PatentedMaylS, 19.71 3,51 3

3 Sheets-Sheet 2 AMBIENT NOISE I l l i l l i GATE ourv W ..STORA6EDlFFERENT/ATOR SAWTOOTH SWEEP .g (30) i OSCILLATOR I (24) i 56 iAMPLIFIER (26) l l I i GATE INVENTOR. HERBERTARASCHKE F G. 2 BY {Mama t!cumsanc A TTORNEYS Pat ted *Ma ls, 1971 3,579,233I

3 Sheets-Sheet 5 I OSCILLATOR LOW FREQUENCY 68T OSCILLATOR I COMPARATOR64 BI STABLE AND STORAGE MULTIVIBRATOR I HIGH BANDPAS'S I I FILTER LowBANDPASS FILTER h h n VARIABLE FREQUENCY 174 OSCILLATOR STO RAG E;

60\ HIGH BANDPAS s FILTER LOW BANDPASS FILTER HERBERT A. RASCHKE INVENTOR.

BY' 7 loumsend loLunsend AMBIENT NOISE FREQUENCY RESPONSIVE AUDIBLEVEHICLE ALARM v This application is a continuation-in-part applicationof the copending Pat. application bearing Ser. No. 571,779, filed Aug.11, 1966, for Audible Vehicle Alarm With Ambient Noise Compensation.

This invention relates to an audible alarm for vehicles and,

more particularly, to such alarm that produces sn output signal having afrequency which differs from the frequency of the ambient noise in whichthe alarm is operated.

In order to afford safety to men employed about motor vehicles such astrucks, fork lift trucks, tractors and the like, mechanical and/orelectrical alarm devices have been provided in the prior art. Suchdevices typically include a sounder or noise making device that producesa constant level signal when the vehicle is moved; Although such devicesare satisfactory when operated in a particular ambient noise environmentfor which they are designed, they are not altogether satisfacto ry whenoperated in-an environment having a greater or lesser ambient noiselevel than the noise level for which they are designed. If the ambientnoise level is less than that for which the alarm is designed, the soundproduced by the alarm is unnecessarily loud and bothersome to workmen.On the other hand, if the alarm is operated in an environment having agreater ambient noise level than that for which the alarm is designed,the workmen are likely not to hear the alarm and thus, are not warned ofpossible danger.

The present invention overcomes the disadvantages to which allusion hasbeen made hereinabove by providing an audible alarm system having anambient noise sensor and a system for varying the output level of thealarm sounder in proportion to the sensed ambient noise level.Therefore, an alarm signal is produced which has a level high enough tobe heard so as to warn the workmen and which is no louder than necessaryto provide such warning. Thus, safety without creation of anunnecessarily bothersome noise level is provided at all times byapparatus of the present invention.

The present invention provides a system that compensates for the factthat the frequency of ambient noise varies from one environment toanother. For example, the frequency constituents of ambient noisepresent at a construction site at which dump trucks and bulldozers arein operation are different from the frequency constituents of ambientnoise in a factory or assembly plant. The present invention provides, incombination with the above mentioned ambient noise sensor, an alarmsignal generator that generates a varying frequency output signal. Ithas been found that by employing a warning signal having a varyingfrequency, the amount by which the level of the warning signal mustexceed the level of the ambient noise is considerably reduced.Accordingly, the frequency of the alarm signal generator may becontinuously varied without reference to the frequency of the ambientnoise so that during each cycle, at least some of the frequenciesgenerated are distinct from the frequencies found in the ambient noise.Alternatively, the frequency of the alarm signal generator may be variedin response to the audio frequencies detected in the ambient noise.

The specific embodiments of the invention to be described in more detailhereinafter include electroacoustic transducers and switching systemsfor operating the transducers on a time shared basis between an ambientnoise sensing function and an alarm sounding function. During the periodthat the electroacoustic transducer is performing the sensing function,an electric signal having an amplitude proportional to the ambient noiselevel is generated and stored, and during the period that theelectroacoustic transducer functions as an alarm sounder, the amplitudeof the stored signal is used to adjust the gain of an amplifier inproportion to the sensed ambient noise level. The amplifier may be fedby a variable frequency oscillator so that it has as an output signal anaudio tone of constantly varying frequency and of an amplitudeproportional to and greater than the ambient noise.

Alternatively, means are provided for sensing the frequency of theambient noise and generating a signal which is fed to the amplifier andhas a frequency different from that of the ambient noise or, at least,from the frequency component which represents the major portion of theambient noise level. The

latter situation arises whenever the ambient noise covers-a relativelybroad frequency spectrum instead of a single frequency or a narrowfrequency band. Broad frequencyspectrums may be encountered in placeswhere machinery emitting high frequency noise, such as metal workingmachinery, and low frequency muffled internal combustion engines arepresent. In practically all instances one of the noise emitters willcontribute the major portion, e.g. more than 50percent of the ambientnoise level. For the purposes of this specification and the appendedclaims frequency different (or distinct, etc.) from the frequency of theambient noise" means, additionally, a frequency difi'erent (or distinct,etc.) from the frequency of the major portion of the ambient noise."

The object, features and advantages of the present invenindicatesschematically a housing for the device which is mounted in a suitablelocation on the vehicle with which the alarm is used. Mounted on thecasing in audio communication with the exterior thereof is abidirectional electroacoustic transducer 14, such as a permanent magnetspeaker. "As is conventional, electroacoustic transducer l4, includes anacoustic structure such as a lightweight cone and an electric structuresuch as a voice coil. The cone and coil are so linked with one anotherthat the cone will vibrate in accordance with the frequency of electricsignals supplied to the voice coil and the voice coil will generate anelectric signal corresponding to vibratory excitation of the cone.Connected to the electric structure of transducer 14 of a gate 16 thatis adapted to couple to the transducer an alarm signal; gate 16 will bereferred to hereinafter as a transmit gate. Also connected to theelectric structure of transducer '14 is a second gate 28 which isadapted to be energized when transducer 14 serves as a sensor forambient noise; gate I8 will hereinafter be referred to as a receivegate.

Transmit gate 116 and receive gate 18 are conventional and gates havingplural inputs and a single output which conveys a signal only when allinputs are energized For energizing the gates on a time shared basis afree-running, bistable multivibrator 20 is provided. Bistablemultivibrator 20 is a conventional circuit that develops an electricaloutput signal having in alternation one or the other of two levels, thechange in level recurring at periodic intervals. The bistablemultivibrator has an output terminal 22 connected to transmit gate 16and a complemental or inverted output terminal 22 connected to the inputor receive gate 18. As is conventional, in a bistable multivibratorcircuits output terrninal'22 is the inverse of output terminal 22' Thusthe gates 16 and 18 are alternatively connected to transducer 14 andcooperate therewith on a time shared basis.

During the time interval that transmit gate 16 is conductive, an alarmsound is delivered to the environment from transducer 14 by excitationof the input thereof from the gate. For generating the alarm signal anaudio oscillator 24 is connected through a variable gain amplifier 26 tothe input of gate 16. Oscillator 24 and amplifier 26 are conventionalcircuit elements. The audio oscillator is capable of delivering avariable frequency output depending for example on the magnitude of thevoltage at an input control terminal 28 thereof. To such input terminalis connected the output of a triggered sawtooth generator 30, aconventional circuit element wellwithin the purview of the skilledartisan. The output of sawtooth generator 30, when the sawtoothgenerator is triggered, is a voltage signal of time-related increasingvoltage, which voltage causes audio oscillator to produce an audiosignal of correspondingly increasing frequency. Sawtooth generator 30 istriggered each time output 22 of a multivibrator increases from arelatively low voltage state to a relatively high voltage state. Thepositive going portion of such output of the multivibrator is connectedthrough a differentiator circuit 32 to input terminal 34 of the sawtoothgenerator. Consequently, each time the multivibrator 20 activatestransmit gate 16, sawtooth generator 30 is pulsed and delivers itsoutput signal to oscillator 24.

The ambient noise in the environment with which transducer 14communicates is conveyed through receive gate 18 each time the output22' of multivibrator 20 is in the relative positive state. Such noiselevel is transmitted through the gate and is amplified by aconventional'amplifier 36. The level of the output of amplifier 36 isproportional to the intensity of ambient noise sensed by transducer 14and such output is stored in a storage element 38, such as a storagecapacitor. Storage element 38 is adapted to store the output ofamplifier 36 for a time period equal to the period of the control signalproduced by multivibrator 20. Such stored signal issupplied to controlterminal 40 of variable gain amplifier 26, the latter amplifier being aconventional element and being adapted to amplify the signal applied toits input by an amount proportional to the level of signal applied tocontrol terminal 40. Thus the gain of amplifier 26 is proportional tothe ambient noise sensed by transducer 14.

A more specific appreciation of the operation of the present inventioncan be had by referring to FIG. 2. The uppermost curve 42 of HG. 2indicates ambient noise which typically includes a period of moderateintensity noise 44, a period of relative quietude 46, and a period ofextremely loud noise 48. Transducer 14 senses such noise intermediateperiods of excitation of the transducer from transmit gate 16, duringwhich intermediate periods receive gate 18 is turned on or activated bymultivibrator output 22'. Thus at the output of receive gate 18 arerepetitive intervals wherein the output of the gate is proportional tothe level of ambient noise. Such intervals are stored in storage element38 in the form of a voltage level, the voltage level of moderate noisebeing designated as 44a, the voltage level of relative quietude beingdesignated at and the voltage level of extremely loud noise beingindicated at 48a. Since the output of storage element 38 is connected tothe gain control input terminal 40 of amplifier 26, the gain of thatamplifier corresponds to the level of the stored signal. Thus when theamplifier is supplied with a signal from oscillator 24, the outputthereof, and consequently the output of transducer 14, will correspondto and be louder than the ambient noise level.

For energizing the audio oscillator, differentiator 32 is provided togenerate a pulse each time transmit gate 16 is turned on to theconductive state by the multivibrator. Such pulse is indicated at 50 andis connected to input terminal 34 of sawtooth generator 30. Each timethe pulse occurs the sawtooth generator produces a voltage signal 52which increases generally linearly with time. Because oscillator 24 is avariable frequency oscillator, i.e., an oscillator in which the outputfrequency is dependent upon the level of voltage applied to controlterminal 28, the audio signal output of oscillator 24 is swept from arelatively low frequency, corresponding to the beginning of the sawtoothwave 52, to a relatively high audio frequency, corresponding to thelater part of sawtooth wave 52. Such variable frequency output signal isindicated at 54. The audio signal is amplified by amplifier 26 by anamount proportional to the voltage level stored in storage element 38,which in turn, is proportional to the ambient noise sensed. Thevariation of the output level of amplifier 26 is depicted graphically at56. Each time transmit gate 16 is turned on to the conductive state bymultivibrator 20, such amplified audio signal As can be seen themagnitude of the warning signal is proportional to the level of ambientnoise present.

Apparatus according to the present invention is preferably designed toproduce an alarm signal that is approximately 5 db. (decibel) above thelevel of the ambient noise sensed. Since ambient noise can reach a levelof db. or more at certain work sites amplifier 26 is designed to drivetransducer 14 to produce a maximum sound level of about db. or more. Byway of example, a large grader pr like earth moving vehicle might verywell produce noise having an intensity of 105 db. A l 10 db. alarmsignal can be heard over such ambient noise level.

An alarm signal intensity of about 8090 db. is not painful or undulystartling'to men working in a relatively quiet en vironment, and so itispreferred that the minimum alarm signal produced by transducer 14 bysuch level irrespective of the amount by which the ambient noise isbelow such level. Many techniques for securing such minimum alarm signalwill occur to those skilled in the art, for example, by application of afixed bias voltage on input tenninal 40 of amplifier 26 to which thevoltage stored in storage element 38 is added when an alarm signal above8090 db. is required.

The loudness of the alarm signal produced by apparatus of "the presentinvention not only exceeds the level of ambient signal is alwaysproduced that has frequency components' distinct from the frequencycomponents of the ambient noise. For example, it has been found that anaudio frequency varying from about 200 cycles per second to about 3,000cycles per second contains sufficient frequency components to bedistinct from virtually any ambient noise condition. Provision of anaudio oscillator 24 capable of such range is well within the capabilityof a skilled artisan.

In one alarm system designed according to the present invention theabove-mentioned frequency variation is accomplished in one-half second,which time period is'determined by the switching or repetition rate ofmultivibrator 20. ln such exemplary system the multivibrator issymmetrical by which is meant the output thereof turns on transmit gate16 for a period equal to and in alternation with the period that receivegate 18 is energized. In other words, the system is designed to senseambient noise for one-half second and to generate an alarm signal duringthesubsequent one-half second period. Variation of the repetition rateof the output of multivibrator 20 can be readily accomplished to altersuch timing sequence and for this reason in the exemplary systemdescribed in detail herein the sawtooth wave produced by sawtoothgenerator 30 in response to excitation at input terminal 34 is somewhatin excess of one-half second, as seen most clearly at 52 in P16. 2.

The system shown in FIG. 1, although effective in attaining the desiredresult, contributes to an increase in overall noise level in theenvironment in which it operates. A modification of the invention, shownin FIG. 3, generates an alarm signal at a frequency distinct from thefrequency of the ambient noise; such alarm can be heard withoutincreasing the level thereof over the level of the ambient noise.Consequently, the overall noise level is not increased.

Referring to P16. 3, an ambient noise frequency-responsive alarm device59 is illustrated. The elements of the device which are identical to theelements utilizedin the alternating frequency alarm device illustratedin FIG. 1 have the same reference numerals as in FIG. 1. When receivegate 18 is conducting, electric signals from transducer 14 are amplifiedat 36 and comprise the input signals to parallel high band and low bandpass filters 60 and 62. Band pass filters are well known in the art andtheir detailed construction is therefore not set forth herein. The passbands of filters 60 and 62 are distinct from one another to affordfrequency separation of is applied to transducer 14 and a warning signalis produced. 75 the ambient noise. For example, a filter 62 having apass band of 500-1000 hz and a filter 60 having a pass band of 1000-2000 hz provides a system that affords satisfactory operation in mostindustrial locations. In systems where ambient noise is separated intoonly two bands, as is the case with the system of FIG. 3, filter 60 canbe a high pass filter and filter 62 can be a low pass filter, the cutofffrequency of each being the same. Where, however, the ambient noise isseparated into three or more hands, a corresponding number of uniqueband pass filters is required.

A comparator and storage device 64 receives the output signals from thetwo band pass filters 60, 62 and is in turn coupled to a high frequencyoscillator 66 and a low frequency oscillator 68. The comparator can beany suitable electronic device, such as a bistable multivibrator whichreceives the output signals from the filters and compares their relativeintensities. Depending on which one of the two output signals isstronger, as a result of the majority of the ambient noise level beingin one or the other frequency band, the bistable multivibrator will bein one or the other of its two modes and thereby energize either thehigh or the low frequency oscillator, but not both. Comparator 64 isconnected so that if the output signals from high band pass filter 60have a greater magnitude than the signals from the low band pass filter62, the low frequency oscillator 66 is energized. Alternatively, if theoutput signals from low band pass filter 62 are of a greater magnitudethan the signals from the high band pass filter, then the multivibratoractuates high frequency oscillator 66 and deenergizes the low frequencyoscillator. That is to say, a signal is produced that has a frequencydistinct from the frequency prevailing in the ambient noise.

The output signals from the high or low frequency oscillators 66, 68 arefed through an amplifier l7 and hence to trans mit gate 16. Oscillators66 and 68 are adapted to produce their respective output signals onlywhen the input terminals, 66T and 68T, respectively, are excited bycomparator and storage circuit 64. The frequency produced by highfrequency oscillator is preferably within the frequency band passed byhigh bandpass filter 60; the frequency produced by low frequencyoscillator is preferably within the frequency band passed by lowband-pass filter 62.

Turning now to the operation of the variable frequency audible warningdevice illustrated in FIG. 3, bistable multivibrator 20 cofunctions withtransmit gate 16 and receive gate 18 as described at the beginning ofthis specification. Thus, the two gates are in alternating conductivestates. When receive gate 18 conducts, and transmit gate 16 is opencircuited, the electric signals from transducer 14 are fed to thefilters 60, 62 after amplification by amplifier 36. Each filter removesall signals having a frequency below or above, respectively, of thefilters threshold frequencies so that no input signals received by themfrom the transducer are lost. Depending upon the frequency of theambient noise, or the frequency of the major portion of the ambientnoise level, the output signal of one or the other of the filters willhave greater magnitude. Accordingly, if comparator 64 comprises abistable multivibrator, the multivibrator will be in one or the other ofits two states, and either one or the other of the high and lowfrequency oscillators 66, 68 is energized. The output signals of theoscillators are fed to amplifier 70 but are prevented from reachingtransducer 14 by the open circuited transmit gate 16.

During the next cycle of bistable multivibrator 20, receive gate 18opens and transmit gate 16 closes, whereby the output signals fromamplifier 70 are transmitted to transducer 14. The transducer generatesan audible signal which has a frequency other than the frequency of theambient noise. The magnitude of the audio signal may therefore be belowthe level of the ambient noise while persons within the danger area ofthe vehicle are warned since the different frequency audio signal can beheard and distinguished over the ambient noise.

The bistable multivibrator, which comprises comparator 64, remains inthe state it was in during the previous cycle in which receive gate I8conducted, acts as a storage for the prevailing signal received fromfilters 60, 62, and continues to energize the same oscillator 66 or 68even after receive gate 18 opens and transmit gate 16 conducts. Signalsof the desired frequency are thus transmitted to transducer 14 each timetransmit gate 16 conducts.

Frequency-responsive warning device 59 may include means to increase thesignal fed to transducer 14 in proportion to the ambient noise level.Amplifier 70 is then selected to be a variable-gain amplifier, which issuitably controlled in response to the ambient noise level as by storageelement 38 shown in FIG. I. In all other respects thefrequency-responsive alarm device 59 is as described in the precedingparagraphs.

Referring to FIG. 4, an ambient noise frequency-responsive warningdevice 72 is constructed to enable the variation of the audio frequencyoutput of transducer 14 over a broad frequency spectrum. Device 72 issimilar to the frequencyresponsive warning device 59 illustrated in theFIG. 3 but includes a single variable frequency oscillator 74 to whichthe output from comparator 76 is fed. Variable frequency oscillator 74is a conventional circuit that produces an AC output signal which has afrequency corresponding to the magnitude of the voltage applied to itsinput terminal 74T from comparator 76. Comparator 76 is electricallycoupled with the high bandand low band-pass filters 60 and 62, which inturn receive their input from transducer 14 via receive gate I8 andamplifier 36.

Comparator 76 can be of any suitable construction, and can comprise thebistable multivibrator of comparator 64 of warning device 59. Thevariable frequency oscillator is connected to one of the outputterminals (not shown) of the multivibrator and receives voltage signalsof a magnitude which vary with the relative intensity of the outputsignals from the bandpass filters. The output signal from the variablefrequency oscillator has a frequency other than the predominantfrequency of the ambient noise; such output is fed to transducer 14through amplifier 78 and transmit gate 16. Amplifier 78 can be avariable gain amplifier, in which case storage element 80, such as astorage capacitor, is provided, which provides the amplifier with acontrol signal fed to the storage element from the output side ofamplifier 36, such control signal having a magnitude proportional to themagnitude of ambient noise.

Warning device 72 operates identically to warning device 59 except thatit provides an audio signal which is variable within the frequencyspectrum of the variable frequency oscillator 74. The variation iseffected by adjusting the output voltage levels of comparator 76.

Although the systems described in connection with FIGS. 3 and 4 providetwo distinct output signals, the invention is not to be considered asbeing limited to that number. For example, a system of the type shown inFIG. 3 can be adapted to divide ambient noise into three or morefrequency bands and to produce alarm signals at three or morefrequencies. Given the circuit descriptions hereinabove set forth, theskilled artisan can construct systems according to the invention thatproduce alarm signals at any reasonable number of distinct frequencies.Thus, it will be seen that the present invention provides an alarmsystem that is audible to workmen in virtually all ambient noiseenvironments. The alarm signal produced may be held to a level justsufficient to be audible above the ambient noise, thereby avoidingexcessively tiresome or startling signals. The variablefrequency-responsive warning device of the present invention, however,permits the noise level of the warning device to be held below theambient noise level, since audible signals of a frequency which differsfrom the frequency of the ambient noise are clearly audible. Because thesystem employs conventional and well-known circuit elements, it iscapable of adjustment to accommodate specialized ambient noiseenvironments which may be encountered in particular applications.

Although several embodiments of the present invention have been shownand described, it will be apparent that other adaptations andmodifications can be made without departing from the true spirit andscope of the invention.

lclaim:

l. Audible signal generating apparatus for a vehicle operated in thepresence of ambient noise comprising means including an electroacoustictransducer for generating an audible signal, means sensing the frequencyof the ambient noise, and electric signal generating means electricallycoupled to said transducer and responsive to said frequencysensingmeans, said signal generating means providing first electric signals asa function of the sensed frequency of the ambient noise causing saidtransducer to emit an audible signal responsive tothe ambient noise andincluding at least one audio frequency component distinct from thefrequency of the ambient noise:

2. Apparatus according to claim 1 wherein the generating means compriseselectric frequency comparing means having a first outputsignaliresponsive to the frequency distribution of the ambientnoise;.and frequency-generating means electrically coupled to saidcomparing means and said transducer, and having a secondoutput signal ofa frequency distinct from the frequency of theambient noise level.

3. Apparatus according to claim 2 wherein said frequencygenerating meanslc'ompn'ses means having a high frequency output and means having a lowfrequency output, and wherein said comparing means energizes one of saidhigh and low frequency output means.

4. Apparatus according to claim 3, including means electrically coupledto said comparing means and to said transducer and separating secondelectric signals from said frequency sensing means into high and lowband frequency electric signals, and wherein said comparing means isresponsive to the relative magnitude of said high and low frequencysignals.

5. Apparatus according to claim 1 wherein said frequency sensing meansprovides input signals for said signal generating means which areresponsive to the relative intensity of the frequencies detected in theambient noise, wherein said electric signal generating means includesmeans having a high audio frequency output, means having a low audiofrequency output, means separating high frequency input signals from lowfrequency input signals, and means measuring the relative intensity ofthe high and low frequency inputs and actuating said means having thehigh audio frequency output in response to a greater intensity of thelow frequency input signals, as compared with the high frequency inputsignals, and the means having the low audio frequency output in responseto greater intensity ofthe high frequency input signals, as comparedwith the low frequency input signals, whereby the audible signalgenerated by said transducer has a frequency range distinct from thefrequency of the ambient noise.

6. Apparatus according to claim 1 wherein the electric signal generatingmeans includes means for generating variable frequency first electricsignals, and means responsive to the frequency of signals received fromthe said frequencysensing means for varying the frequency of saidvariable frequency signal generating means so that the frequency of thesignals from the variable frequency signal generating means differs fromthe frequency of the ambient noise.

1. Audible signal generating apparatus for a vehicle operated in thepresence of ambient noise comprising means including an electroacoustictransducer for generating an audible signal, means sensing the frequencyof the ambient noise, and electric signal generating means electricallycoupled to said transducer and responsive to said frequency-sensingmeans, said signal generating means providing first electric signals asa function of the sensed frequency of the ambient noise causing saidtransducer to emit an audible signal responsive to the ambient noise andincluding at least one audio frequency component distinct from thefrequency of the ambient noise.
 2. Apparatus according to claim 1wherein the generating means comprises electric frequency comparingmeans having a first output signal responsive to the frequencydistribution of the ambient noise, and frequency-generating meanselectrically coupled to said comparing means and said transducer, andhaving a second output signal of a frequency distinct from the frequencyof the ambient noise level.
 3. Apparatus according to claim 2 whereinsaid frequency-generating means comprises means having a high frequencyoutput and means having a low frequency output, and wherein saidcomparing means energizEs one of said high and low frequency outputmeans.
 4. Apparatus according to claim 3, including means electricallycoupled to said comparing means and to said transducer and separatingsecond electric signals from said frequency sensing means into high andlow band frequency electric signals, and wherein said comparing means isresponsive to the relative magnitude of said high and low frequencysignals.
 5. Apparatus according to claim 1 wherein said frequencysensing means provides input signals for said signal generating meanswhich are responsive to the relative intensity of the frequenciesdetected in the ambient noise, wherein said electric signal generatingmeans includes means having a high audio frequency output, means havinga low audio frequency output, means separating high frequency inputsignals from low frequency input signals, and means measuring therelative intensity of the high and low frequency inputs and actuatingsaid means having the high audio frequency output in response to agreater intensity of the low frequency input signals, as compared withthe high frequency input signals, and the means having the low audiofrequency output in response to greater intensity of the high frequencyinput signals, as compared with the low frequency input signals, wherebythe audible signal generated by said transducer has a frequency rangedistinct from the frequency of the ambient noise.
 6. Apparatus accordingto claim 1 wherein the electric signal generating means includes meansfor generating variable frequency first electric signals, and meansresponsive to the frequency of signals received from the saidfrequency-sensing means for varying the frequency of said variablefrequency signal generating means so that the frequency of the signalsfrom the variable frequency signal generating means differs from thefrequency of the ambient noise.